Ultrasonic receiving circuit

ABSTRACT

An ultrasonic occupancy sensor for detecting presence or absence of an occupant in a space includes an ultrasonic receiving circuit having a synchronous rectifier that allows the circuit to detect small-magnitude ultrasonic waves having a Doppler shift. The sensor comprises an ultrasonic transmitter for transmitting ultrasonic waves at an ultrasonic operating frequency, and a controller that drives the transmitting circuit with complementary drive signals to control the ultrasonic operating frequency. The synchronous rectifier receives the drive signals from the controller and rectifies an amplified input signal to generate a rectified signal, which is filtered by a filter to generate a filtered signal. The controller receives the filtered signal and determines that the space is occupied if the magnitude of the filtered signal exceeds a threshold. The ultrasonic occupancy sensor may also include a low phase-noise oscillator circuit coupled to the controller for setting an internal operating frequency of the controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application of commonly-assignedU.S. Provisional Application Ser. No. 61/316,011, filed Mar. 22, 2010,entitled ULTRASONIC RECEIVING CIRCUIT, the entire disclosure of which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to occupancy and vacancy sensors fordetecting an occupancy or a vacancy in a space, and more particularly,to an ultrasonic receiving circuit for an occupancy or vacancy sensor.

2. Description of the Related Art

Occupancy and vacancy sensors are often used to detect occupancy and/orvacancy conditions in a space in order to control an electrical load,such as, for example, a lighting load. An occupancy sensor typicallyoperates to turn on the lighting load when the occupancy sensor detectsthe presence of a user in the space (i.e., an occupancy event) and thento turn off the lighting load when the occupancy sensor detects that theuser has left the space (i.e., a vacancy event). A vacancy sensor onlyoperates to turn off the lighting load when the vacancy sensor detects avacancy in the space. Therefore, when using a vacancy sensor, thelighting load must be turned on manually (e.g., in response to a manualactuation of a control actuator).

Occupancy and vacancy sensors have often been provided in wall-mountedload control devices that are coupled between an alternating-current(AC) power source and an electrical load for control of the amount ofpower delivered to the electrical load. Some prior art occupancy andvacancy sensors have been provided as part of lighting control systems.These sensors are often coupled via a wired control link to a lightingcontroller (e.g., a central processor), which then controls the lightingloads accordingly. Alternatively, the sensors may be battery-powered andmay be operable to transmit wireless signals, such as radio-frequency(RF) signals, to load control devices, such as dimmer switches. Theseoccupancy and vacancy sensors are not required to be mounted inelectrical wallboxes, but may be mounted to the ceiling or high on awall. Therefore, the occupancy and vacancy sensors may be positionedoptimally to detect the presence of the user in all areas of the space.

The prior art occupancy and vacancy sensors typically comprise internaldetectors, such as, for example, a pyroelectric infrared (PIR) detector,and a lens for directing energy to the PIR detector for detecting thepresence of the user in the space. Alternatively, some prior artoccupancy and vacancy sensors have included ultrasonic transmitting andreceiving circuits for detecting the presence of the user in the space.Ultrasonic sensors transmit ultrasonic waves at a predeterminedfrequency, and analyze received ultrasonic waves to determine if thereis an occupant in the space. The received ultrasonic waves that arereflected off of moving objects will be characterized by a Doppler shiftwith respect to the transmitted ultrasonic waves, while the receivedultrasonic waves that are produced by reflections off of the walls,ceiling, floor, and other stationary objects of the room will not have aDoppler shift. Therefore, ultrasonic occupancy and vacancy sensors areable to determine if there is an occupant in the space if there is aDoppler shift between the frequencies of the transmitted and receivedultrasonic waves.

Generally, the size of the objects that produce the ultrasonic waveshaving the Doppler shift (such as, a moving hand) are very small, andthus produce reflected ultrasonic waves having small magnitudes. One ofthe issues with detecting ultrasonic waves having a Doppler shift isthat these received ultrasonic waves can be difficult to distinguishfrom the received ultrasonic waves that do not have a Doppler shift. Afigure of merit for occupancy detection limits can be described usingthe signal-to-interference ratio (SIR), which is the ratio of theDoppler-shifted ultrasonic waves expressed in sound pressure level (SPL)to the non-Doppler-shifted ultrasonic waves.

The standard implementations for detecting Doppler shifts in ultrasonicwaves take three standard forms. The first form uses a phase-lock-loop(PLL) integrated circuit (IC), such as, for example, part numberCD74HC7046, manufactured by Texas Instruments Incorporated. In thisimplementation, the received ultrasonic waves are amplified by apre-amplifier and then compared with a single fixed threshold (e.g., 100mV) using a comparator to yield a binary waveform. The binary waveformis then applied to an exclusive-or (XOR) gate where the second input tothe XOR is a clock input (e.g., a 40-kHz clock signal) that also drivesthe ultrasonic transmitting circuit. The resulting signal is then passedthrough a band-pass filter to extract the Doppler signal. The resultingDoppler signal is then compared to a fixed threshold using anothercomparator to detect an occupancy or vacancy condition. The drawback ofthis implementation is that the circuit is very sensitive to thethresholds of the comparators and only works on signals with an SIRgreater than approximately −40 dB.

The second prior art form for detecting Doppler shift implements thedetection algorithm primarily within a microcontroller. In thisimplementation, the received ultrasonic waves are amplified by apreamplifier and then sampled using an analog-to-digital converter(e.g., an 8 to 12-bit ADC) in the microcontroller. The remainder of thealgorithm is essentially the same as in the first prior art form fordetecting Doppler shift described above except that the remainder of thealgorithm of the second form is executed in the software of themicrocontroller. This implementation depends on the accuracy of the ADCof the microcontroller and is limited by numerical noise due to the ADCquantization and the numerical precision used to calculate the results,which thus limits the ability to detect small-magnitude ultrasonic wavesthat have a Doppler shift.

The third prior art form is an amplitude-modulation (AM) demodulator,which, in its simplest form, uses a diode and a low-pass filter to forman envelope detector. The limitation of this circuit is that thereceived ultrasonic signal must have a minimum amplitude to render thediode conductive, thus reducing the ability of the circuit to detectsmall-magnitude ultrasonic waves that have a Doppler shift.

Thus, there is a need for a simple, effective low-cost ultrasonicreceiving circuit for use in occupancy or vacancy sensors that avoidsthe disadvantages of the prior art.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, an ultrasonicreceiving circuit for detecting the presence or absence of an occupantin a space comprises: (1) an ultrasonic receiving element for generatingan ultrasonic input signal; (2) an amplifier circuit for amplifying theinput signal to generate an amplified signal; (3) a synchronousrectifier for rectifying the input signal to generate a rectifiedsignal; and (4) a filter for filtering the rectified signal to generatea filtered signal, where the filtered signal indicates the presence ofthe occupant in the space if the magnitude of the filtered signalexceeds a threshold. The synchronous rectifier may comprise anon-inverting amplifier and an inverting amplifier both receiving theamplified signal. The synchronous rectifier may further comprise a firstanalog switch coupled to an output of the non-inverting amplifier and asecond analog switch coupled to an output of the inverting amplifier.The first and second analog switches may be electrically coupledtogether, such that the outputs of the analog switches may be mixedtogether to form the rectified signal.

An ultrasonic occupancy sensor for detecting presence or absence of anoccupant in a space is also described herein. The occupancy sensorcomprises an ultrasonic transmitting circuit having an ultrasonictransmitting element for transmitting ultrasonic waves, and anultrasonic receiving circuit having an ultrasonic receiving element forgenerating an ultrasonic input signal. The ultrasonic receiving circuitfurther comprises an amplifier circuit for amplifying the input signalto generate an amplified signal, a synchronous rectifier for rectifyingthe amplified signal to generate a rectified signal, and a filter forfiltering the rectified signal to generate a filtered signal. Thesynchronous rectifier may comprise a non-inverting amplifier and aninverting amplifier both receiving the amplified signal. The synchronousrectifier may further comprise a first analog switch coupled to anoutput of the non-inverting amplifier and a second analog switch coupledto an output of the inverting amplifier. The first and second analogswitches may be electrically coupled together for forming the rectifiedsignal. The occupancy sensor further comprises a controller operable todrive the ultrasonic transmitting circuit with a drive signal to controlan ultrasonic transmission frequency of the transmitted ultrasonicwaves. The controller may be operable to render the first and secondanalog switches conductive and non-conductive on a complementary basis,such the outputs of the analog switches may be mixed together to formthe rectified signal. The controller receives the filtered signal anddetermines that the space is occupied if a magnitude of the filteredsignal exceeds a threshold.

According to another embodiment of the present invention, the controllermay drive the ultrasonic transmitting circuit with complementary drivesignals to control an ultrasonic transmission frequency of thetransmitted ultrasonic waves. The synchronous rectifier may also receivethe drive signals from the controller, such that the filtered signalrises above the upper threshold or falls below the lower threshold whenthe frequency of the received ultrasonic waves is different than thefrequency of the transmitted ultrasonic waves. The controller may beoperable to determine that the space is occupied if the magnitude of thefiltered signal rises above an upper threshold or falls below a lowerthreshold. In addition, the ultrasonic occupancy sensor may furthercomprise a low phase-noise oscillator circuit coupled to the controllerfor setting an internal operating frequency of the controller. Theultrasonic transmission frequency of the transmitted ultrasonic wavesmay be approximately equal to the internal operating frequency of thecontroller.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in the followingdetailed description with reference to the drawings in which:

FIG. 1 is a simple diagram of a radio-frequency (RF) lighting controlsystem having a dimmer switch and an ultrasonic occupancy sensor forcontrolling the amount of power delivered to a lighting load accordingto an embodiment of the present invention;

FIG. 2 is a simplified block diagram of the ultrasonic occupancy sensorthat allows for detecting the presence or absence of an occupant in aspace surrounding the lighting load of the lighting control system ofFIG. 1;

FIG. 3A shows example waveforms illustrating the operation of anultrasonic receiving circuit of the occupancy sensor of FIG. 2 whenthere is not an occupant in the space;

FIG. 3B shows an example waveform that illustrates the operation of theultrasonic receiving circuit of the occupancy sensor of FIG. 2 whenthere is an occupant in the space;

FIG. 4 is a simplified circuit diagram of a portion of the ultrasonicreceiving circuit of the occupancy sensor of FIG. 2 showing a clampcircuit and a non-linear amplifier in detail;

FIG. 5 is a simplified circuit diagram of a portion of the ultrasonicreceiving circuit of the occupancy sensor of FIG. 2 showing asynchronous rectifier in detail;

FIG. 6 is a simplified circuit diagram of a portion of the ultrasonicreceiving circuit of the occupancy sensor of FIG. 2 showing a bandpassfilter in detail; and

FIG. 7 is a simplified flowchart of a control procedure executedperiodically by a controller of the occupancy sensor of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofthe preferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purposes of illustrating theinvention, there is shown in the drawings an embodiment that ispresently preferred, in which like numerals represent similar partsthroughout the several views of the drawings, it being understood,however, that the invention is not limited to the specific methods andinstrumentalities disclosed.

FIG. 1 is a simple diagram of a radio-frequency (RF) lighting controlsystem 100 comprising a dimmer switch 110 and a remote ultrasonicoccupancy sensor 120 according to an embodiment of the presentinvention. The dimmer switch 110 is adapted to be coupled in serieselectrical connection between an AC power source 102 and a lighting load104 for controlling the amount of power delivered to the lighting load.The dimmer switch 110 may be adapted to be wall-mounted in a standardelectrical wallbox. Alternatively, the dimmer switch 110 could beimplemented as a table-top load control device. The dimmer switch 110comprises a faceplate 112 and a bezel 113 received in an opening of thefaceplate. The dimmer switch 110 further comprises a toggle actuator114, i.e., a button, and an intensity adjustment actuator 116.Successive actuations of the toggle actuator 114 toggle, i.e., turn offand on, the lighting load 104. Actuations of an upper portion 116A or alower portion 116B of the intensity adjustment actuator 116 respectivelyincrease or decrease the amount of power delivered to the lighting load104 and thus increase or decrease the intensity of the lighting load 104from a minimum intensity (e.g., 1%) to a maximum intensity (e.g., 100%).A plurality of visual indicators 118, e.g., light-emitting diodes(LEDs), are arranged in a linear array on the left side of the bezel113. The visual indicators 118 are illuminated to provide feedback ofthe intensity of the lighting load 104. An example of a dimmer switchhaving a toggle actuator 114 and an intensity adjustment actuator 116 isdescribed in greater detail in U.S. Pat. No. 5,248,919, issued Sep. 29,1993, entitled LIGHTING CONTROL DEVICE, the entire disclosure of whichis hereby incorporated by reference. Examples of a dimmer switchoperable to transmit and receive digital messages is described ingreater detail in U.S. patent application No. 12/033,223, filed Feb. 19,2008, entitled COMMUNICATION PROTOCOL FOR A RADIO-FREQUENCY LOAD CONTROLSYSTEM, the entire disclosure of which is hereby incorporated byreference.

The occupancy sensor 120 may be removably mountable to a ceiling or awall, for example, in the vicinity of (i.e., a space around) thelighting load 104 controlled by the dimmer switch 110, such that theoccupancy sensor is operable to detect an occupancy condition (i.e., thepresence of the occupant) or a vacancy condition (i.e., the absence ofthe occupant) in the vicinity of the lighting load. The lighting controlsystem 100 may comprise additional occupancy sensors 120 that are spacedapart so as to detect occupancy conditions in different areas of thevicinity of the lighting load 104. The occupancy sensor 120 is operableto transmit digital messages wirelessly via RF signals 106 to the dimmerswitch 110 in response to detecting occupancy conditions or vacancyconditions in the space around the lighting load 104. The occupancysensor 120 may be battery-powered, or may be connected to an externalpower source.

According to the embodiment of the present invention, the occupancysensor 120 has an occupancy sensing circuit comprising an internalultrasonic transceiver, which is housed in an enclosure 122 and includesan ultrasonic transmitting element Ul (FIG. 2) and an ultrasonicreceiving element U2 (FIG. 2). The enclosure 122 has a vent 124 forallowing ultrasonic waves to be transmitted by the ultrasonictransmitting element Ul and received by the ultrasonic receiving elementU2. The occupancy sensor 120 is operable to determine whether occupancyconditions or vacancy conditions are presently occurring in the space inresponse to the ultrasonic waves received by the ultrasonic receivingelement U2 as will be described in greater detail below. Alternatively,the occupancy sensor 120 could additionally comprise a passive infrared(PIR) detector, a microwave detector, or any suitable detector orcombination of detectors.

During a setup procedure of the RF lighting control system 100, thedimmer switch 110 may be assigned to (i.e., associated with) theoccupancy sensor 120 (and to additional occupancy sensors). The setupand configuration of a lighting control system including occupancysensors is described in greater detail in U.S. patent application No.12/371,027, filed Feb. 13, 2009, entitled METHOD AND APPARATUS FORCONFIGURING A WIRELESS SENSOR, the entire disclosure of which is herebyincorporated by reference.

A message transmitted by the occupancy sensor 120 may include a commandand indentifying information, for example, a 52-bit serial number (i.e.,a unique identifier) associated with the transmitting occupancy sensor.The dimmer switch 110 is responsive to messages containing the serialnumbers of the occupancy sensors 120 to which the dimmer switch isassigned. The commands included in the digital messages transmitted bythe occupancy sensor 120 may comprise an occupied command or a vacantcommand. When the lighting load 104 is off, the dimmer switch 110 isoperable to turn on the lighting load in response to receiving a firstoccupied command from any one of the occupancy sensors 120 to which thedimmer switch is assigned. The dimmer switch 110 is operable to turn offthe lighting load 104 in response to the last vacant command receivedfrom those occupancy sensors 120 from which the occupancy sensorreceived occupied commands. For example, if two occupancy sensors 120both transmit occupied commands to the dimmer switch 110, the dimmerswitch will not turn off the lighting load 104 until subsequent vacantcommands are received from both of the occupancy sensors. The operationof a lighting control system including wireless occupancy sensors isdescribed is greater detail in U.S. patent application No. 12/203,518,filed Sep. 3, 2008, entitled RADIO-FREQUENCY LIGHTING CONTROL SYSTEMWITH OCCUPANCY SENSING, the entire disclosure of which is herebyincorporated by reference.

FIG. 2 is a simplified block diagram of the occupancy sensor 120 of thelighting control system 100. FIG. 3A shows example waveformsillustrating the operation of the ultrasonic occupancy sensor 120 whenthere is not an occupant in the space, and FIG. 3B shows an examplewaveform illustrating the operation of the occupancy sensor 120 whenthere is an occupant in the space. The occupancy sensor 120 comprises acontroller 210 coupled to an ultrasonic transmitting circuit 220 fortransmitting the ultrasonic waves and an ultrasonic receiving circuit230 for receiving the ultrasonic waves to thus detect the presence andabsence of the occupant in the space. For example, the controller 210may be a microprocessor, but may alternatively be any suitableprocessing device, such as a programmable logic device (PLD), amicrocontroller, an application specific integrated circuit (ASIC), or afield-programmable gate array (FPGA). The ultrasonic transmittingcircuit 220 comprises the ultrasonic transmitting element Ul and theultrasonic receiving circuit 230 comprises the ultrasonic receivingelement U2. The ultrasonic transmitting element U1 and the ultrasonicreceiving element U2 may both comprise, for example, piezoelectricelements.

The controller 210 is coupled to a low phase-noise oscillator circuit212 for setting an internal operating frequency f_(op) of the controller(e.g. approximately 40 kHz). The low phase-noise oscillator circuit 212may comprise, for example, a Pierce oscillator circuit as shown in FIG.2 having a crystal 214, such as a 40-kHz piezoelectric crystal, e.g.,part number CM250C, manufactured by Citizen Crystal. For example, thelow phase-noise oscillator circuit 212 may be characterized by aspectral purity of approximately -60 dBc at 5 Hz from the ratedfrequency (i.e., 40 kHz). The low phase-noise oscillator circuit 212further comprises an inverter 215, two capacitors C216, C217 (e.g., eachhaving a capacitance of approximately 12 pF), and two resistors R218,R219 (e.g., having resistances of approximately 10 MΩand 392 kΩ,respectively). Alternatively, the low phase-noise oscillator circuit 212could comprise any suitable external low phase-noise oscillator circuit,or an internal low phase-noise oscillator circuit of the controller 210.

The occupancy sensor 120 comprises a battery V1 that generates adirect-current (DC) supply voltage V_(CC1) (e.g., approximately 3 volts)for powering the controller 210 and the ultrasonic receiving circuit230. The occupancy sensor 120 further comprises a boost converter 240that receives the supply voltage V_(CC1) and generates a boosted voltageV_(CC2) (e.g., approximately 12 volts) for powering the ultrasonictransmitting circuit 220. The controller 210 is further coupled to amemory 250 for storage of operating characteristics of the occupancysensor 120. The memory 250 may be implemented as an external integratedcircuit (IC) or as an internal circuit of the controller 210. Theoccupancy sensor 120 further comprises an RF transmitter 260 coupled tothe controller 210 and an antenna 262 for transmitting the RF signals106. Alternatively, the occupancy sensor 120 could comprise an RFtransceiver to allow for two-way communication between the occupancysensor and the dimmer switch 110.

The ultrasonic transmitting circuit 220 may comprise a standard H-bridgedrive circuit 222 for energizing the piezoelectric element (i.e., theultrasonic transmitter U1). Specifically, the controller 210 drives theH-bridge drive circuit 222 with a non-inverted ultrasonic drive signalV_(DRIVE) and an inverted ultrasonic drive signal V_(DR—INV), which mayboth comprise complementary square wave signals as shown in FIG. 3A. Thecontroller 210 generates the non-inverted ultrasonic drive signalV_(DRIVE) and the inverted ultrasonic drive signal V_(DR—INV) such thatboth signals are characterized by an ultrasonic transmission frequencyf_(US), which may be equal to the operating frequency f_(OP) of thecontroller (i.e., approximately 40 kHz). Since the operating frequencyf_(OP) of the controller 210 is derived from the low phase-noiseoscillator circuit 212, the non-inverted ultrasonic drive signalV_(DRIVE) and an inverted ultrasonic drive signal V_(DR—INV), and thusthe ultrasonic transmission frequency f_(US), are also characterized bylow phase noise.

The ultrasonic receiver U2 of the ultrasonic receiving circuit 230generates a received ultrasonic input signal V_(IN) in response to thereceived ultrasonic waves. The input signal V_(IN) is clamped by a clampcircuit 232 to protect the other circuitry of the ultrasonic receivingcircuit 230. A non-linear amplifier 234 receives the input signal V_(IN)and generates an amplified signal V_(AMP). A gain G_(NL) of thenon-linear amplifier 234 is approximately 11 when the magnitude of theAC component of the input signal V_(IN) is small (e.g., less thanapproximately 1.2 volts), and approximately 2 when the magnitude of theAC component of the input signal V_(IN) is large (e.g., greater thanapproximately 1.2 volts). The ultrasonic receiving circuit 230 furthercomprises a synchronous rectifier 236 (i.e., a lock-in amplifier), whichreceives the amplified signal V_(AMP) from the non-linear amplifier 234and generates a rectified signal V_(RECT). The synchronous rectifier 236also receives the non-inverted ultrasonic drive signal V_(DRIVE) and theinverted ultrasonic drive signal V_(DR—INV) from the controller 210.

A bandpass filter 238 (e.g., an anti-aliasing filter) generates afiltered signal V_(FILT) from the rectified signal V_(RECT) and has abandwidth of approximately 5-100 Hz. The controller 210 receives thefiltered signal _(VFILT) from the bandpass filter 238 and includes, forexample, an analog-to-digital converter (ADC) for sampling the filteredsignal. The controller 210 is operable to detect the presence of theoccupant in the space if the magnitude of the filtered signal risesabove an upper voltage threshold V_(TH+) (e.g., approximately 0.25volts) or falls below a lower voltage threshold V_(TH−) (as shown inFIG. 3B). Since the filtered signal V_(FILT) is biased to approximatelyone-half of the supply voltage V_(CC1) (i.e., approximately 1.5 volts),the filtered signal _(VFILT) will have a DC magnitude equal toapproximately 1.5 volts and will remain between the upper voltagethreshold V_(TH+) and the lower voltage threshold V_(TH−) if there isnot an occupant in the space (as shown in FIG. 3A). The upper voltagethreshold V_(TH+) and the lower voltage threshold V_(TH−) may bepredetermined fixed values or may be adjustable by the controller 210.In addition, the controller 210 may be operable to digitally filter thefiltered signal V_(FILT) received from the bandpass filter 238 toprovide additional filtering of the signal before determining if thespace is occupied or unoccupied.

According to the embodiment of the present invention, the synchronousrectifier 236 eliminates many of the limitations of the prior art. Thesynchronous rectifier 236 can work on input signals having very smallmagnitudes and is not limited by an ADC and numerical precisions of amicrocontroller. Design principles and testing have demonstrated thatsignals with an SIR of −80 dB can easily be detected using thesynchronous rectifier 236 of the ultrasonic receiving circuit 230 of theembodiment of the present invention.

FIG. 4 is a simplified circuit diagram of a portion of the ultrasonicreceiving circuit 230 showing the clamp circuit 232 and the non-linearamplifier 234 in greater detail. The clamp circuit 232 comprises aresistor R310 coupled between the coupled between the output of theultrasonic receiver U2 and circuit common, a first diode D312 coupledbetween the output of the ultrasonic receiver U2 and the supply voltageV_(CC1), and a second diode D314 coupled between circuit common and theoutput of the ultrasonic receiver U2. Accordingly, the magnitude of theinput voltage V_(IN) is prevented from rising above a diode drop (e.g.approximately 0.7 volts) above the magnitude of the supply voltageV_(CC1), and from dropping below a diode drop below the circuit common.

The non-linear amplifier circuit 234 comprises an operational amplifier(op amp) U314. The input voltage V_(IN) is coupled to the non-invertinginput of the op amp U314 via a capacitor C316 and a resistor R318. Acircuit (comprising resistors R320, R322, R324 and a capacitor C326) iscoupled to the junction of the capacitor C316 and the resistor R318, andoperates to reference the input voltage V_(IN) at one-half of the supplyvoltage V_(CC1). The inverting input of the op amp U314 is coupled tocircuit common through a resistor R328 and a capacitor C330, and to theoutput via a resistor R332. The non-linear amplifier circuit 234 alsocomprises a resistor R344 and a plurality of diodes D340, D342, D344,D346 coupled between the inverting input and the output of the op ampU314.

When the magnitude of the AC component of the input signal V_(IN) issmall (i.e., less than approximately 1.2 volts), the diodes D340, D342,D344, D346 are non-conductive and only the resistor R332 is coupledbetween the inverting input and the output of the op amp U314, such thatthe gain G_(NL) of the non-linear amplifier 234 is approximately 11.When the magnitude of the AC component of the input signal V_(IN) islarge (i.e., greater than approximately 1.2 volts), the either thediodes D340, D342 or the diodes D344, D346 become conductive and theresistor R334 is coupled in parallel with the resistor R332. As aresult, the equivalent resistance coupled between the inverting inputand the output of the op amp U314 decreases and the gain G_(NL) of thenon-linear amplifier 234 decreases to approximately 2. Specifically,when the magnitude of the voltage at the output of the op amp U314 isless than the magnitude of the voltage at the inverting input byapproximately the sum of the forward voltages of the diodes D340, D342(i.e., two diode drops, e.g., approximately 1.2 volts), the diodes D340,D342 are conductive. When the magnitude of the voltage at the output ofthe op amp U314 is greater than the magnitude of the voltage at theinverting input by approximately two diode drops, the diodes D344, D346are conductive. Accordingly, the non-linear amplifier 234 amplifies theinput signal V_(IN) by a greater amount when the magnitude of the inputsignal V_(IN) is small than when the magnitude of the input signalV_(IN) is large to thus generate the amplified signal V_(AMP).

FIG. 5 is a simplified circuit diagram of a portion of the ultrasonicreceiving circuit 230 showing the synchronous rectifier 236 in greaterdetail. The synchronous rectifier 236 comprises a non-invertingamplifier 410 and an inverting amplifier 412 that both receive theamplified signal V_(AMP) from the non-linear amplifier 234.Specifically, the amplified signal V_(AMP) is coupled to thenon-inverting amplifier 410 via a capacitor C420 and is further coupledto the inverting amplifier 412 via a capacitor C422. A circuit(comprising resistors R424, R426, R428 and a capacitor C430) is coupledto the junction of the capacitors C440, C442, and operates to referencethe amplified signal V_(AMP) at one-half of the supply voltage V_(CC1).

The non-inverting amplifier 410 comprises an op amp U432 and has a gainG_(NI) of approximately 10. The inverting input of the op amp U432 iscoupled to circuit common through a resistor R434 and a capacitor C436,and is coupled to the output through a resistor R438. The invertingamplifier 412 comprises an op amp U440 and has a gain G_(I) ofapproximately −10. The inverting input of the op amp U440 is coupled tothe capacitor C422 via a resistor R442 and to the output via a resistorR444. The non-inverting input of the op amp U440 is coupled to a circuitthat comprises resistors R446, R448 and a capacitor C450 and operates tobias non-inverting input at one-half of the supply voltage V_(CC1).

The outputs of the non-inverting amplifier 410 and the invertingamplifier 412 are coupled to respective analog switches S414, S416,which are rendered conductive and non-conductive on a complementarybasis in response to the non-inverted ultrasonic drive signal V_(DRIVE)and the inverted ultrasonic drive signal V_(DR—INV), respectively. Theoutputs of the analog switches S414, S416 are mixed together to form therectified signal V_(RECT) as shown in FIG. 3A. When the non-invertedultrasonic drive signal V_(DRIVE) is high (i.e., approximately equal tothe supply voltage V_(CC1)) and the first switch S414 is conductive, therectified signal V_(RECT) is equal to the output of the non-invertingamplifier 410. When the inverted ultrasonic drive signal V_(DR—INV) ishigh and the second switch S416 is conductive, the rectified signalV_(RECT) is equal to the output of the inverting amplifier 412.

FIG. 6 is a simplified circuit diagram of a portion of the ultrasonicreceiving circuit 230 showing the bandpass filter 238 in greater detail.The bandpass filter 238 comprises a low-pass filter stage 510 and anon-inverting amplifier stage 512. The low-pass filter stage 510includes an op amp U520 having a non-inverting input coupled to receivethe rectified voltage V_(RECT) through two resistors R522, R524. Thejunction of the two resistors R522, R524 is coupled to circuit commonthrough a resistor R525. The non-inverting input is also coupled tocircuit common via a capacitor C526. The output of the op amp U520 iscoupled directly to the inverting input and is coupled to the junctionof the resistors R522, R524 through a capacitor C528. The output of thelow-pass filter stage 510 is coupled to the non-inverting amplifierstage 512 via a circuit that includes two capacitors C530, C538, andthree resistors R532, R534, R536. The low-pass filter stage 510 operatesto filter out the 80-kHz components of the rectified voltage V_(RECT).

The non-inverting amplifier stage 512 comprises an op amp U540 and has again G_(BP) of approximately 100. The non-inverting input of the op ampU540 is coupled to the output of the low-pass filter stage 510 via thecapacitor C530 and the resistor R536. The inverting input of the op ampU540 is coupled to circuit common through a resistor R542 and acapacitor C544, and is coupled to the output via a resistor R546. Acapacitor C548 is also coupled between the inverting input and theoutput of the op amp U540 and provides additional low-pass filtering.The filtered voltage V_(FILT) is generated from the output of the op ampU540 via a circuit that comprises two capacitors C550, C558 and threeresistors R552, R554, R556. The resistors R552, R554 reference thefiltered voltage V_(FILT) at one-half of the supply voltage V_(CC1) asshown in FIG. 3A. The capacitors C526, C544, C550 provide high-passfiltering for the bandpass filter 238. Accordingly, the bandpass filter238 has a band width of 5-100 Hz.

The controller 210 receives the filtered voltage V_(FILT) and determinesthe presence or absence of the occupant in the space in response to themagnitude of the filtered voltage. Since the ultrasonic transmissionfrequency f_(US) of the transmitted waves is dependent upon thenon-inverted ultrasonic drive signal V_(DRIVE) and the invertedultrasonic drive signal V_(DR—INV) and the analog switches S414, S416 ofthe synchronous rectifier 236 are controlled in response to thenon-inverted ultrasonic drive signal V_(DRIVE) and the invertedultrasonic drive signal V_(DR—INV), the synchronous rectifier circuitwill properly rectify the amplified signal V_(AMP) and the filteredvoltage V_(FILT) stays between the upper voltage threshold V_(TH+) andthe lower voltage threshold V_(TH−) when there is not an occupant in thespace. However, if there is an occupant in the space, there will be aDoppler shift in the received ultrasonic waves as compared to thetransmitted ultrasonic waves. In addition, the frequency of the receivedultrasonic waves will be varying with respect to time (i.e., thedifference between frequencies of the received ultrasonic waves and theultrasonic transmission frequency f_(US) of the transmitted waves willvary with respect to time). Accordingly, rectified signal V_(RECT)generated by the synchronous rectifier circuit 236 will vary inmagnitude and the filtered voltage V_(FILT) will become greater than theupper voltage threshold V_(TH+) and or less than the lower voltagethreshold V_(TH−), thus signally that the space is occupied.

FIG. 7 is a simplified flowchart of a control procedure 600 executedperiodically by the controller 210 (e.g., approximately every two msec).The controller 210 first samples the filtered signal V_(FILT) togenerate a sample S at step 410. At step 612, the controller 210 removesthe DC component of the sample S by subtracting one-half of the supplyvoltage V_(CC1) from the sample (i.e., S_(R) =S−V_(CC1)/2). Next, thecontroller 210 determines the magnitude (i.e., the absolute value) ofthe sample S (i.e., S_(ABS) =|S_(R)|) at step 614. If the absolute valueS_(ABS) is greater than or equal to a sample threshold S_(TH) at step616, the controller 210 determines that the space is occupied at step618 and the control procedure 600 exits. If the absolute value S_(ABS)is less than the sample threshold S_(TH) at step 616, the controller 210determines that the space is vacant at step 620, before the controlprocedure 600 exits. For example, the sample threshold S_(TH) may beequal to approximately one-half of the difference between the uppervoltage threshold V_(TH+)and the lower voltage threshold V_(TH−) shownin FIGS. 3A and 3B, i.e., S_(TH) =(V_(TH+)−V_(TH−))/2.

In addition, the occupancy sensor 120 could additionally comprise asecond occupancy sensing circuit including, for example, a passiveinfrared (PIR) detector, such that the occupancy sensor is a“dual-technology” occupancy sensor. Since the PIR detector usesdifferent technology than the ultrasonic transmitting circuit 220 andthe ultrasonic receiving circuit 230, the dual-technology occupancysensor provides for an increased ability to detect the presence of anoccupant in the space surrounding the occupancy sensor. An example of anoccupancy sensing circuit having a PIR detector is described in greaterdetail in U.S. patent application No. 12/203,500, filed Sep. 3, 2008,entitled BATTERY-POWERED OCCUPANCY SENSOR, the entire disclosure ofwhich is hereby incorporated by reference.

While the present invention has been described with reference to thedimmer switch 110 for controlling the intensity of the lighting load104, the concepts of the present invention could be applied to loadcontrol systems comprising other types of load control devices, such as,for example, fan-speed controls for fan motors, temperature controldevices for control of heating, ventilation, and air-conditioning (HVAC)systems, electronic dimming ballasts for fluorescent loads, and driversfor light-emitting diodes (LEDs). Further, the concepts of the presentinvention could be used to control other types of electrical loads, suchas, for example, fan motors or motorized window treatments.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

The values provided herein for the values and part numbers of thecomponents of FIGS. 4, 5, and 6 are provided as examples only and shouldnot limit the scope of the present invention. For example, it would bewell within the capabilities of one having ordinary skill in the art tomodify the values of the components of FIGS. 4, 5, and 6 and stillobtain the ultrasonic receiving circuit of the present invention.

What is claimed is:
 1. An ultrasonic occupancy sensor for detectingpresence or absence of an occupant in a space, the occupancy sensorcomprising: an ultrasonic transmitting circuit comprising an ultrasonictransmitting element for transmitting ultrasonic waves; an ultrasonicreceiving circuit comprising an ultrasonic receiving element forreceiving ultrasonic waves and generating an ultrasonic input signal, anamplifier circuit for amplifying the amplified signal to generate anamplified signal, a synchronous rectifier for rectifying the inputsignal to generate a rectified signal, and a filter for filtering therectified signal to generate a filtered signal, the synchronousrectifier comprising a non-inverting amplifier and an invertingamplifier both receiving the amplified signal, the synchronous rectifierfurther comprising a first analog switch coupled to an output of thenon-inverting amplifier and a second analog switch coupled to an outputof the inverting amplifier, the first and second analog switcheselectrically coupled together for forming the rectified signal; and acontroller operable to drive the ultrasonic transmitting circuit with adrive signal to control an ultrasonic transmission frequency of thetransmitted ultrasonic waves, the controller operable to render thefirst and second analog switches conductive and non-conductive on acomplementary basis such the outputs of the analog switches are mixedtogether to form the rectified signal, the controller further operableto receive the filtered signal and to determine that the space isoccupied if a magnitude of the filtered signal exceeds a threshold. 2.The sensor of claim 1, wherein the synchronous rectifier of theultrasonic receiving circuit also receives the drive signal from thecontroller, such that the filtered signal exceeds the threshold when areceived ultrasonic frequency of the received ultrasonic waves isdifferent than the ultrasonic transmission frequency of the transmittedultrasonic waves.
 3. The sensor of claim 2, further comprising: a lowphase-noise oscillator circuit coupled to the controller for setting aninternal operating frequency of the controller.
 4. The sensor of claim3, wherein the ultrasonic transmission frequency of the transmittedultrasonic waves is approximately equal to the internal operatingfrequency of the controller.
 5. The sensor of claim 3, wherein the lowphase-noise oscillator circuit comprises Pierce oscillator circuithaving a crystal.
 6. The sensor of claim 2, wherein the ultrasonicreceiving element and the ultrasonic transmitting element both comprisepiezoelectric elements.
 7. The sensor of claim 1, wherein the amplifiercircuit comprises a non-linear amplifier operable to amplify the inputsignal when the magnitude of the input signal is small, and not toamplify the input signal when the magnitude of the input signal islarge.
 8. The sensor of claim 1, wherein the filter comprises ananti-aliasing filter, and the controller is operable to digitally filterthe filtered signal from the filter prior to determining if the space isoccupied.
 9. The sensor of claim 1, wherein the controller is operableto determine that the space is occupied if a magnitude of the filteredsignal rises above an upper threshold or falls below a lower threshold.10. An ultrasonic occupancy sensor for detecting presence or absence ofan occupant in a space, the occupancy sensor comprising: an ultrasonictransmitting circuit comprising an ultrasonic transmitting element fortransmitting ultrasonic waves; an ultrasonic receiving circuitcomprising an ultrasonic receiving element for receiving ultrasonicwaves and generating an ultrasonic input signal, an amplifier circuitfor amplifying the input signal to generate an amplified signal, asynchronous rectifier for rectifying the amplified signal to generate arectified signal, and a filter for filtering the rectified signal togenerate a filtered signal; a controller operable to drive theultrasonic transmitting circuit with complementary drive signals tocontrol an ultrasonic transmission frequency of the transmittedultrasonic waves, the controller further operable to receive thefiltered signal and to determine that the space is occupied if amagnitude of the filtered signal rises above an upper threshold or fallsbelow a lower threshold; and a low phase-noise oscillator circuitcoupled to the controller for setting an internal operating frequency ofthe controller, the ultrasonic transmission frequency of the transmittedultrasonic waves being approximately equal to the internal operatingfrequency of the controller; wherein the synchronous rectifier alsoreceives the drive signals from the controller, such that the filteredsignal rises above the upper threshold or falls below the lowerthreshold when a received ultrasonic frequency of the receivedultrasonic waves is different than the ultrasonic transmission frequencyof the transmitted ultrasonic waves.
 11. The sensor of claim 10, whereinthe synchronous rectifier comprises a non-inverting amplifier and aninverting amplifier both receiving the amplified signal, the synchronousrectifier further comprising a first analog switch coupled to an outputof the non-inverting amplifier and a second analog switch coupled to anoutput of the inverting amplifier, the first and second analog switcheselectrically coupled together, the controller configured to render thefirst and second analog switches conductive and non-conductive on acomplementary basis such that outputs of the analog switches are mixedtogether to form the rectified signal.
 12. The sensor of claim 10,wherein the low phase-noise oscillator circuit comprises Pierceoscillator circuit having a crystal.
 13. An ultrasonic receiving circuitfor detecting a presence or absence of an occupant in a space comprises:an ultrasonic receiving element for generating an ultrasonic inputsignal; an amplifier circuit for amplifying the input signal to generatean amplified signal; a synchronous rectifier for rectifying theamplified signal to generate a rectified signal, the synchronousrectifier comprising a non-inverting amplifier and an invertingamplifier both receiving the amplified signal, the synchronous rectifierfurther comprising a first analog switch coupled to an output of thenon-inverting amplifier and a second analog switch coupled to an outputof the inverting amplifier, the first and second analog switcheselectrically coupled together such that outputs of the analog switchesare mixed together to form the rectified signal; and a filter forfiltering the rectified signal to generate a filtered signal; whereinthe filtered signal indicates the presence of the occupant in the spaceif the magnitude of the filtered signal exceeds a threshold.
 14. Thecircuit of claim 13, wherein the ultrasonic receiving element receivesultrasonic waves representative of whether the space is occupied orunoccupied, the synchronous rectifier receiving two complementary drivesignals characterized by low phase noise, the filtered signal exceedingthe threshold when the frequency of the received ultrasonic waves isdifferent than the frequency of the drive signals.
 15. The circuit ofclaim 14, wherein the drive signals comprises square wave voltagescoupled to the analog switches for controlling the analog switches to beconductive and non-conductive on a complementary basis.
 16. The circuitof claim 13, wherein the filtered signal indicates the presence of theoccupant in the space if the magnitude of the filtered signal risesabove an upper threshold or falls below a lower threshold.